Just a robot face
Nebula, although she was always an oddball, has made great strides since Guardians of the Galaxy. Now, she's one of the most likable characters in the MCU, and her journey to befriend Gamora is one of the best things about Endgame.
First is the way she looks on the comics. They show her looking like a giant Luphomoid, with blue and purple skin. However, she was a completely different creature in the past and had many different incarnations throughout her life.
She was also a pirate by trade, so she enjoyed many adventures throughout the universe. She even fought the Skrulls in some of the more cosmic Marvel stories.
Thanos was Thanos her father in MCU (and she had her as her uncle too), but she wasn’t related to him. During their time together, they got into all kinds of trouble with various enemies -- even though they had no real connection other than the fact that they were both mercenaries.
Her relationship with Thanos was a complicated one, and it ended up becoming her biggest enemy. Despite being loyal to her adopted father she became resentful towards him and his constant use her as a weapon in the war against others.
Nebula's resentment grew. She was forced by Gamora to compete with her in combat. Thanos would also maim her and implant robotic enhancements into a part her body. This resentment only intensified as she grew older and started to realize how much she didn't like Thanos.
While she eventually made amends with her sister and was able make peace with Thanos, she still had difficulty with her adoption and the things Thanos did. She became a reluctant ally for the Guardians after turning against Thanos.
It is obvious that she was an extremely powerful character in MCU. She was also the first Guardian to join in the film. She is also a key character in Endgame and one of the most emotional.
Surprisingly, and even unexpectedly, her transformation to a hero is quite remarkable. It's also quite possible that she will be the final death of Thanos, as she claims to do at the end of the film.
How she became a Cyborg
Nebula, an alien with purple and blue skin was the original Nebula in the MCU. However, Thanos' evil mind made her a cyborg. Her condition led to her being subjected a variety of surgeries, bionic and other bizarre enhancements.
How she changed during MCU
Guardians 2 Nebula tells of Thanos using her and Gamora to punish one another in fighting matches. He would take away a part of their bodies and replaced it with some sort of robotic enhancement, making them both look pretty awful.